If someone is using Parallels 11, 12 or 13 and VirtualBox 5 - it is much much easier.
Step 1: Expand the Package Contents of the parallel's HDD
Step 2: Within that there is an HDS file, ideally there will be only 1 file containing the data (unless split option was used), just double check the size of the file which is a good indication of the file you are looking for.
Step 3: Copy the HDS file to a new location that VirtualBox can access, rename the file and change its extension to hdd.
Step 4: Attach the newly copied file directly with the VirtualBox.
Step 5: Test everything, this is good to go.
Step 6: Run the guest OS, uninstall old parallels tool and install VirtualBox guest tools.
These steps will get finished within minutes and no conversion needed.
I have just finished executing all of these steps day before and tested.
After this, I executed the below listed Optional steps, so that the hdd file gets converted into VDI file. With VDI file the VirtualBox gets more control: most important one being optimizing the free space, which i sorely needed.
Optional Steps: (Switch off guest OS before this)
Step 7: From within VirtualBox tool, Open Global Tools, it will list all harddisks
Step 8: Use Copy function, this will export the attached HDD file to any other compatible options: VDI, VHD, VMDK and more, after researching which is best option i chose VDI - since this is native to VirtualBox.
In future if i need to export the Virtual machine to some other format, then VirtualBox has export option and supports "Open Virtualisation Format".
Basically with the above procedure, you skip the parallels tool for conversion, somewhere VMware converter was proposed, you skip that too. You will only need 1 single tool: VirtualBox, to complete all the steps.